FIELD OF THE INVENTION
This invention is related generally to the field of hydrodynamic bearings, and especially to the field of rotating shaft motors utilizing fluid dynamic bearings.
Disk drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disk medium, the actual information being stored in the form of magnetic transitions within the medium. The disks themselves are rotatably mounted on a spindle, the information being accessed by means of transducers located on a pivoting arm, which moves radially over the surface of the disk. The read/write heads or transducers must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information; thus the disks must be rotationally stable.
During operation, the disks are rotated at very high speeds within an enclosed housing by means of an electric motor, which is generally located inside the hub or below the disks. One type of motor in common use is known as an in-hub or in-spindle motor. Such in-spindle motors typically have a spindle mounted by means of two ball bearing systems to a motor shaft disposed in the center of the hub. One of the bearings is typically located near the top of the spindle, and the other near the bottom. These bearings allow for rotational movement between the shaft and hub, while maintaining accurate alignment of the spindle to the shaft. The bearings themselves are normally lubricated by grease or oil.
The conventional bearing system described above, however, is prone to several shortcomings. First is the problem of vibration generated by the balls rolling on the raceways. Ball bearings used in hard disk drive spindles run under conditions that generally guarantee physical contact between raceway and ball, in spite of the lubrication layer provided by the bearing oil or grease. Hence, bearing balls running on the generally smooth but microscopically uneven and rough raceways transmit this surface structure as well as their imperfection in sphericity in the form of vibration to the rotating disk. This vibration results in misalignment between the data tracks and the read/write transducer. This source of vibration limits the data track density and the overall performance of the disk drive system.
Another problem is related to the application of hard disk drives in portable computer equipment, resulting in severely increased requirements for shock resistance. Shocks create relative acceleration between the disks and the drive casing, which in turn show up as a force across the bearing system. Since the contact surfaces in ball bearings are very small, the resulting contact pressures may exceed the yield strength of the bearing material, and leave long-term deformation and damage to the raceway and the balls of the ball bearing.
Moreover, mechanical bearings are not easily scaleable to smaller dimensions. This is a significant drawback since the tendency in the disk drive industry has been to continually shrink the physical dimensions of the disk drive unit.
As an alternative to conventional ball bearing spindle systems, researchers have concentrated much of their efforts on developing a hydrodynamic bearing. In these types of systems, lubricating fluidxe2x80x94either gas or liquidxe2x80x94functions as the actual bearing surface between a stationary base or housing and the rotating spindle or rotating hub of the motor. For example, liquid lubricants comprising oil, more complex ferro-magnetic fluids or even air have been utilized in hydrodynamic bearing systems. The reason for the desirability of the use of air is the importance of avoiding the outgassing of contaminants into the sealed area of the head/disk housing. However, air does not provide the lubricating qualities of oil. The relatively higher viscosity of oil allows for larger bearing gaps and therefore looser tolerance standards to achieve similar dynamic performance.
A common type of fluid dynamic bearing comprises a shaft extending through the sleeve or hub with one or more radially extending plates supported from the shaft. A hydrodynamic bearing is provided between the shaft and the bore through the hub, with the fluid, which occupies the gap between the inner surface of the bore and the outer surface of the shaft providing the stiffness for the shaft. Without this stiffness, the shaft is prone to tilting or wobbling over the life of the motor. As a result, any hub or disk supported for rotation by the shaft is prone to wobbling or tilting. Any such tilting or instability in the hub or disk would make reading or writing of data on the disk surface very difficult, and diminish the life of the motor and the disk drive in which it is used.
However, the very fact that a fluid dynamic bearing design relies on the use of fluid in a very narrow gap between a shaft and surrounding bore for establishing and maintaining radial stiffness and fluid in a gap between a thrust plate and the surrounding bore to maintain axial stiffness creates a problem. A fluid of an extremely high viscosity, or even air will not provide the desired stiffness, damping, and loading capacity. Therefore, the obvious direction would be to go to thicker, heavier, higher viscosity lubricants to fill the gap and stabilize the system. However, power loss in a high speed liquid lubricant bearing is also a major concern. The use of a thicker lubricant would create a drag; therefore, the problem remains of achieving a balance between providing the requisite loading damping and stiffness, while minimizing power consumption required to cause rotation in the system.
The present invention relates to a fluid dynamic bearing in which in addition to the journal and/or thrust bearing which is provided to support the shaft for rotation, a supplementary fluid dynamic bearing is provided to enhance the axial and radial stiffness, damping and loading capacity of the design.
According to the present invention, in one embodiment, a motor is disclosed comprising a rotating shaft where the shaft comprises both an axial shaft and at least one thrust plate, both the shaft and thrust plate being supported for rotation by fluid dynamic bearings. A secondary fluid dynamic bearing is provided, coupled to the main support fluid dynamic bearing, and is oriented to provide additional axial and/or radial stiffness. Preferably, the secondary fluid dynamic bearing is defined between the sleeve which establishes the bore wherein the shaft rotates, the sleeve having an outer surface which is cylindrical and defines and includes one surface of the secondary fluid dynamic bearing, the other surface being defined by the inner surface of a hub which is coupled mechanically to the rotating shaft and thrust plate and is rotating over and around the sleeve.
The thrust plate bearing and a part of the journal bearing could preferably comprise fluid; the remainder of the journal bearing and the supplementary or secondary bearing could comprise either a lighter viscosity fluid or even air.
In a form of the invention, the gap of the liquid thrust bearing can be reduced to the smallest possible size to reduce power loss. In a further embodiment, the journal bearing which comprises a liquid bearing could also be substantially diminished in size.
In another approach to the invention, a portion of the journal bearing would comprise a lower viscosity fluid than the remainder of the journal bearing and the thrust bearing, and/or even air.
In another embodiment of the invention, the secondary support bearing would be defined in a gap which is wider than the gap in either the primary journal bearing and/or the thrust bearing.
In another feature of the invention, the fluid in the secondary bearing would only be air or a very light viscosity fluid.
In a further feature of the invention, the grooves in the secondary bearing gap would be spiral, and orient to pump the light viscosity fluid or air only towards the journal bearing, further stabilizing both axially and radially the support bearing for the shaft and thrust plate.